A liquid chromatography/mass spectrometer (LC/MS) is widely used in the analysis of biometric samples, for example. In the ion source of the LC/MS, gaseous ions are generated from a liquid sample separated in the LC, and introduced into amass analyzing unit. For an ionization method in the ion source, a spray ionization method according to electrospray ionization (ESI) is widely used. A capillary that is a tube in the inner diameter generally ranging from about a few μm to a few 100 μm is used between the LC and the ion source of the mass spectrometer. The electrsospray ionization is generally performed at an atmospheric pressure, in which a high voltage is applied across the liquid sample at the end portion of the capillary tubed to the LC and a counter electrode (the inlet of the mass analyzing unit), and charged droplets are generated by the electrostatic spraying phenomenon. The generated charged droplets are evaporated, and gaseous ions are generated. The efficiency of generating gaseous ions is more increased, as the size of the first generated charged droplet is smaller and the charge amount is higher.
In electrospray ionization in these years, nano-electrospray is being conducted in which the inner diameter of a capillary used for introducing a sample is downscaled to about 100 μm to about 1 to 2 μm in order to reduce the flow velocity of liquid sample. The nano-electrospray allows the measurement of a small amount of a sample for a longer time, and a small amount of biometric molecules can be analyzed.
Because of the advent of the nano-spray, the flow velocity of liquid sample for ionization is roughly divided into nano-spray at a few 100 nL/minute or less and the flow velocity at a few tens μL/minute or greater, which is the micro flow velocity. In the case of a fast flow velocity of liquid sample at a few tens μL/minute or greater, a heated spray gas is generally used in order to promote the desolvation of a liquid sample.